1. Filed of the Invention
The present invention relates to a method for measuring a 6 degrees of freedom (DOF) motion of a rigid body such as vehicle, and a device therefor.
2. Prior Art
In recent years, due to diversified types and enhanced performances of vehicles, the measurement and evaluation of human vibrations and vibration intervals, which provide criteria of how comfortable a rider in a car feels or so-called driving feeling (e.g. sense of maneuverability, stability, braking ability and the like), are becoming increasingly important. Particularly, it is widely expected that the evaluation relative to a wider range of subjects such as beginner drivers, aged persons, and so on will become increasingly important hereafter. For such evaluation, it is necessary to measure a motion of a moving vehicle with a sufficient band width (i.e., in the range of up to about DC 20 Hz).
Conventionally, a three-axis linear accelerometer and a three-axis mechanical rate gyroscope have been recognized as measurement devices for measurement of 6 degrees of freedom (DOF) motion of a rigid body such as vehicle. However, for a vehicle which performs such a complex motion of 6 DOF, the center of rotational motion is not constant, and acceleration signals to be detected by the linear accelerometer involve not only linear acceleration but also centrifugal and angular effects, which, therefore, must be separated therefrom. Whilst angular velocity data or angular acceleration data are necessary for such separation, a conventional rate gyroscope has such a low response characteristic that a measurable range has been limited to a motion of low speed, and its phase characteristics also is different from that of linear accelerometer, so that it has a drawback that it is not suitable for separation of signals.
Further, due to limited installing positions of such conventional measurement devices for real vehicles, coordinate transformation such as the transformation of measured linear acceleration and angular acceleration into arbitrary position or posture has often been necessary. For such coordinate transformation, the measured linear acceleration and angular acceleration must be combined with each other, for which, however, a conventional rate gyroscope is not considered suitable for the same reasons discussed above.
To solve the above-mentioned problems, there is proposed one method or device for measurement of angular acceleration (angular velocity) without the use of rate gyroscope, in which several (nine, for example) one-axis accelerometers are combined one another three-dimensionally. With such measurement method, however, there are problems such that the device becomes large-sized, requiring the accuracy of disposition of each one-axis accelerometer, and that the algorithm for analysis of output signals from the one-axis accelerometers becomes complicated.
Also, there are proposed other devices for measurement of this kind, some of the representatives of which are disclosed in Japanese Examined Patent Publication No. 6-90218 and Japanese Registered Patent Publication No. 2622297.
In the No. 6-90218 publication is proposed a device which comprises a casing and six support legs for restraining an overall 6 DOF motion of an inertia body, each support leg being disposed between the casing and the inertia body, having a force-detector mounted thereto, thus detecting urging forces applied to each leg thereby to measure all the accelerations of 6 DOF. However, as each leg is disposed three-dimensionally in a space inside the casing, the above-mentioned problems, such as the large-sized device and the requirement for accuracy of disposition, are unavoidable. In addition, as an inertia body is shifted by the forces from external due to the expansion or shrinkage of the legs, there would be structurally developed the Coriolis force. Nevertheless, as the outputs from the six force-detectors are simply converted into linear matrix for analysis, disregarding the Coriolis force and centrifugal effects when processing, you would be unable to measure accelerations other than slight acceleration and angular acceleration (or angular velocity). This is due to the fact that an inertia body in a three-dimensional space is subjected not only to inertial forces derived from accelerations and angular accelerations, but also to the Coriolis force and centrifugal forces, which act upon one another in a very complex manner, thus causing errors. Accordingly, you cannot expect the accurate measurement of 6 DOF motion in a three-dimensional space without taking such causes for errors into consideration.
On the other hand, in the No. 2622297 publication is proposed a device which comprises: first and second radial magnetic bearings and a pair of thrust magnetic bearings, said bearings holding in place a rotational body which rotates around the X axis; a detector for detecting the number of rotation of the rotational body; and five displacement-detectors for detecting the displacement of the rotational body, said displacement being defined in the directions of the Y, Z axes and the thrust, respectively, whereby the displacement acceleration and displacement angular acceleration of the rotational body are calculated and output, while correcting the displacement in the radial and thrust directions of the rotational body. Although the principle disclosed in this publication is analogous to the one disclosed in the No. 6-90218 publication, the structure becomes more complicated due to the presence of the rotational body. Whilst the prior art also teaches that the detection by the displacement-detectors may be performed, taking advantage of the Coriolis force which is developed when an angular velocity is developed around the Y axis of the rotational body, such method would result either in the incomplete separation of the Coriolis force and centrifugal effects, or in the dispersion of the output signals, and thus stable measurement would be impossible.
In other words, the conventional six-axis accelerometers have been structured without the sufficient analyses of the relationships of the acceleration and angular acceleration relative to an inertia body, even under the condition where the Coriolis force and centrifugal effects are developed structurally. Therefore, although they would work well relative to an inertia body moving on a two-dimensional plane, they, in a three-dimensional space, are unable to completely remove the effects of the Coriolis force and centrifugal forces which interfere complexly in the direction of each axis, or otherwise, they allow the output signals to be dispersed, and thus there is a likelihood that no stable measurement is able to be performed.